Each optional parameter has a default value as its part of the definition. If no argument is sent for the optional parameter, default value is being used. Also, default value of the optional parameter must be constant and optional parameter must define at the end of the all required parameter. It can be applied on constructor, method, and indexer.

C# 4.0 allows a new static type called “dynamic”. Any operation on the object of type dynamic resolve at runtime. So, it gets escaped from compile type checking. But if any error, it caught at runtime. Also, the dynamic type allows us to access the object, without knowing type of the object at compile time. It causes no intelli-sense support.

Marks the next character as either a special character or escapes a literal. For example, “n” matches the character “n”. “\n” matches a newline character. The sequence “\\” matches “\” and “\(” matches “(“.

Note: double quotes may be escaped by doubling them: “<a href=””…>”

^

Depending on whether the MultiLine option is set, matches the position before the first character in a line, or the first character in the string.

$

Depending on whether the MultiLine option is set, matches the position after the last character in a line, or the last character in the string.

*

Matches the preceding character zero or more times. For
example, “zo*” matches either “z” or “zoo”.

+

Matches the preceding character one or more times. For
example, “zo+” matches “zoo” but not “z”.

?

Matches the preceding character zero or one time. For
example, “a?ve?” matches the “ve” in “never”.

.

Matches any single character except a newline character.

(pattern)

Matches pattern and remembers the match. The matched substring can be retrieved from the resulting Matches collection, using Item [0]…[n]. To match parentheses characters
( ), use “\(” or “\)”.

n is a non-negative integer. Matches exactly n times.
For example, “o{2}” does not match the “o” in “Bob,” but matches the
first two o’s in “foooood”.

{n,}

n is a non-negative integer. Matches at least n times.
For example, “o{2,}” does not match the “o” in “Bob” and matches all
the o’s in “foooood.” “o{1,}” is equivalent to “o+”. “o{0,}” is equivalent
to “o*”.

{n,m}

m and n are non-negative integers. Matches
at least n and at most m times. For example, “o{1,3}” matches
the first three o’s in “fooooood.” “o{0,1}” is equivalent to “o?”.

[xyz]

A character set. Matches any one of the enclosed characters.
For example, “[abc]” matches the “a” in “plain”.

[^xyz]

A negative character set. Matches any character not enclosed.
For example, “[^abc]” matches the “p” in “plain”.

[a-z]

A range of characters. Matches any character in the specified
range. For example, “[a-z]” matches any lowercase alphabetic character
in the range “a” through “z”.

[^m-z]

A negative range characters. Matches any character not
in the specified range. For example, “[m-z]” matches any character not
in the range “m” through “z”.

\b

Matches a word boundary, that is, the position between
a word and a space. For example, “er\b” matches the “er” in “never” but
not the “er” in “verb”.

Matches any word character including underscore. Equivalent
to “[A-Za-z0-9_]”.

\W

Matches any non-word character. Equivalent to “[^A-Za-z0-9_]”.

\num

Matches num, where num is a positive integer. A reference back to remembered matches.
For example, “(.)\1” matches two consecutive identical characters.

\n

Matches n, where n is an octal escape value. Octal escape values must be 1, 2, or 3 digits long.
For example, “\11” and “\011” both match a tab character. “\0011” is the equivalent of “\001” & “1”. Octal escape values must not exceed 256. If they do, only the first two digits comprise the expression. Allows ASCII codes to be used in regular expressions.

\xn

Matches n, where n is a hexadecimal escape value. Hexadecimal escape values must be exactly two digits long.
For example, “\x41” matches “A”. “\x041” is equivalent to “\x04” & “1”.
Allows ASCII codes to be used in regular expressions.

\un

Matches a Unicode character expressed in hexadecimal notation with exactly four numeric digits. “\u0200” matches a space character.

\A

Matches the position before the first character in a string. Not affected by the MultiLine setting

\Z

Matches the position after the last character of a string. Not affected by the MultiLine setting.

\G

Specifies that the matches must be consecutive, without any intervening non-matching characters.